Balance EMPTY, as with most planes, a little nose heavy at takeoff is fine as fuel is
consumed the Center of mass will shift rearwards.
Since the tank is behind the CG, balancing it empty will result in the airplane being tail heavy with the tank full, (at takeoff) and the center of mass will shift forward as the fuel is consumed
It would be best if the instructions told you whether to balance it with the tank empty or full, but of course they don't. Balancing it with the tank full will result in a more stable (noseheavy) condition, but looking at these threads, it seems that people have been able to make it work either way. I balanced mine with the tank half full on the theory that it is better to be off a little than off a lot.
Jim